1. Technical Field
This invention relates to a method and apparatus for countergravity casting molten metal in a mold using an electromagnetic pump.
2. Description Of Related Prior Art
Countergravity casting is often used for producing high quality, thin-walled castings. With all known low pressure systems, a casting mold is supported above a vessel containing a supply of molten metal and some means are provided for delivering the metal against gravity from the vessel into the mold. Low pressure countergravity casting enables a slow, tranquil fill of the mold, assuring that even the very thin sections of the casting will be fully developed.
With some systems, the delivery of metal is effectuated by pressurizing the entire supply of metal in the vessel with air or other gas. Precisely controlling the flow of metal in such systems, however, is difficult since any change is countered by the momentum of the entire metal supply. In other words, the entire supply must react to a change in flow for any portion thereof to react.
Other known low pressure systems utilize an electromagnetic pump rather than pressurized air for delivering molten aluminum metal into the mold. With such systems, the pump is typically accommodated within the vessel and is responsive to changes in input voltage for delivering only a fraction of the metal supply from the vessel into the mold. Since only a small portion of the metal supply is under pressure at any given time, metal momentum is significantly less a factor when desiring to make changes in metal flow. Consequently, rapid and frequent changes can be made to the metal flow for precisely controlling the fill of the mold.
Of those low pressure casting systems known to utilize electromagnetic pumps, the pump is most often accommodated in an open well of the vessel. The open well, however, is a source for a tremendous amount of heat loss as well as contamination of the metal from exposure to the external atmosphere. Aluminum metal both oxidizes and picks up hydrogen when exposed which, if cast into the mold, produces defects within the casting.
To account for the heat loss, these systems are known to heat the metal well above the desired casting temperature which, in turn, produces temperature differences throughout the melt. The temperature variation is harmful to the pump in that it subjects the pump to thermal cycling and shortens its life. These pumps are very costly. It also affects the viscosity and corresponding flow characteristics of the metal. This is problematic in that the characteristic output of the pump changes with changing metal viscosity. Thus, controlling the rate at which metal is pumped into the becomes more difficult.
Another problem with overheating the metal is that aluminum's affinity for hydrogen increases with increasing temperature thereby further adding to the hydrogen contamination of the metal.
One system is known to provide a cover over the well of the vessel for lessening the heat loss and is disclosed in the U.S. Pat. No. 4,967,827 to Campbell, granted Nov. 6, 1990. The cover, however, does not protect the molten metal from contamination by the external atmosphere as the environment in the space between the cover and the molten metal is not taught as being any different from that of the external atmosphere. As such, this system presents all of the problems of contamination as those with no cover.
Accordingly, there is a need in the industry for a low pressure countergravity casting system utilizing an electromagnetic pump which both insulates the molten metal from heat loss as well as protecting it against contamination from the external atmosphere.